Cilia are tiny cellular structures that protrude from cells. They are about 0.25 micrometers in diameter and contain a bundle of microtubules. They are widespread among living organisms, occurring in most animals, many single-celled eukaryotes, and in some lower plants.
Cilia tend to function in one of two ways. They may move fluid across the surface of the cell or they may propel cells through a fluid. They may also serve to gather food. In humans, cilia on the surfaces of respiratory epithelia function to push mucus and trapped particles and dead cells out of the lungs. Cilia also function to carry eggs through the oviduct. Cilia function in myriad ways in different kinds of cells.
Flagella are structures related to cilia. They are similar in internal structure but tend to be much longer than cilia. Sperm cells are propelled by flagella, as are many other single-celled eukaryotes.
Groups of cilia tend to move together in coordinated unidirectional waves. The motion made by each individual cilium is whiplike. This motion includes two phases. First, the cilium extends forward, pushing against the surrounding liquid as it goes. At the end of its forward stroke, the cilium bends, reducing viscous drag as it pulls itself back to its original position. By contrast, flagella tend to propagate quasi-sinusoidal waves. Despite the differences in their external motions, the molecular basis of movement in both cilia and flagella appear to be the same.
Cilia and flagella move by bending their core—the axoneme. The axoneme is composed of microtubules and associated proteins. The pattern of microtubules is distinctive: nine pairs of microtubules that form a ring around two single microtubules. This arrangement is typically referred to as “9+2”. The pairs are composed of one complete and one partial microtubule. These microtubules extend the full length of the axoneme, which can range in length from 10-200 micrometers.
Intraflagellar transport (IFT) is a dynein and kinesin-based motility process in which non-membrane-bound particles move along flagellar microtubules, just beneath the flagellar membrane, from the base to the tip of the flagellum and back. IFT is essential for the assembly and maintenance of all cilia and flagella, including non-motile primary cilia and sensory cilia. Recent results indicate that defects in IFT are a primary cause of several human diseases.